The present invention generally relates to a storage system, and in particular relates to a storage system in which a second storage subsystem is externally connected to a first storage subsystem, and a control method of such a storage system.
In order to deal with a system environment that undergoes sudden changes due to increase in the data amount or data traffic, there is a compound storage system in which a storage subsystem is externally connected to an existing storage subsystem.
This compound storage system, as described in Japanese Patent Laid-Open Publication No. 2006-178811 (Patent Document 1), virtualizes a volume of a first storage subsystem, associates a storage area of a second storage subsystem to the virtualized volume, and transfers the I/O from a host computer to the virtualized volume of the first storage subsystem to the second storage subsystem to store data in the storage area thereof. The storage capacity of the first storage subsystem to be used by the host system can thereby be expanded.
As a control method of a plurality of logical paths defined between the first storage subsystem and the second storage subsystem, Patent Document 1 describes the first storage subsystem using a primary path among the plurality of paths to issue an I/O to the volume of the second storage subsystem.
When a failure occurs in the primary path, the I/O control right, which is the authority for passing the I/O through, is switched from the primary path to other paths in descending order of priority. For reference's sake, the method of transferring I/O via a single path as described above is referred to as a single path mode.
Meanwhile, a round robin method where the first storage subsystem distributively issues the I/O to a plurality of paths between the first storage subsystem and the second storage subsystem is known as a multi-path mode.
Japanese Patent Laid-Open Publication No. 2007-265243 relates to a computer system comprising a host computer and a storage system, and describes technology of switching a logical path connecting the host computer and the storage system.